The present invention relates to a polymer actuator that can suppress degradation even when forced displacement is externally applied, a robot arm driven by the polymer actuator, and a robot equipped with the robot arm.
With increase in demand for machines that operate near humans such as a household robot, expectations for an artificial muscle actuator that operates flexibly as in muscles of a human are also rising. Actuators of various types have been proposed as candidates for the artificial muscle actuator, among them are actuators using conductive polymers, actuators using dielectric polymers, and the like.
An actuator which generates flexural deformation as shown in FIGS. 5A, 5B, and 5C is proposed as one example of the artificial muscle actuator using a conductive polymer. The actuator has a configuration of sandwiching a solid electrolyte molding member 32 with polyaniline film members 35a, 35b serving as conductive polymer films. A potential difference set in a power supply 36 is applied between the polyaniline film members 35a, 35b by turning ON a switch 37, whereby negative ions enter one polyaniline film member 35b thereby extending the relevant film member and negative ions are released from the other polyaniline film member 35a thereby contracting the relevant film member as shown in FIG. 5B, and consequently, flexural deformation occurs (see e.g., patent document 1).
In such configuration, the flexural deformation occurs by the difference in displacement amount of the two conductive polymer film members 35a, 35b serving as electrodes, but an actuator is also known, which has a configuration where the electrolyte retention layer is formed by a liquid or gel substance to prevent deformations of both electrodes from influencing each other, and the displacement of only one of the conductive polymer film members 35a, 35b is taken out to conduct extending and contracting deformation. In this case, the electrode in which displacement is not used does not need to be a conductive polymer, and although metal electrodes are mainly used, it is shown that the displacement may be increased by arranging conductive polymers on the metal electrode (see e.g., non-patent document 1).
Such conductive polymer actuator produces distortion comparable to muscles at a low voltage of 2 to 3V, and thus is expected to be put to practical use as an artificial muscle.
An actuator that utilizes elastic deformation of polymers as shown in FIGS. 6A and 6B is proposed as one example of the artificial muscle actuator using dielectric polymers. The actuator is configured by a dielectric polymer 42 of a flat plate shape, flexible electrodes 41, 43 of a thin film shape made of carbon particles such as graphite or carbon black or metal arranged on both surfaces of the dielectric polymer 42, a power supply 46 connected between the electrodes 41, 43, and a switch 47. When the switch 47 is turned ON and the potential difference set in the power supply 46 is applied between the electrodes 41, 43, the dielectric polymer member 42 compresses, and expands in the lateral direction as shown in FIG. 6B. The dielectric polymer 42 restores to the state of FIG. 6A when the switch 47 is turned OFF.
Such an actuator produces a strain equal to or greater than 100% by using silicon rubber or acrylic for the dielectric polymer 42, and thus is expected to be put to practical use as the artificial muscle (see e.g., non-patent document 2).
However, in the case of the actuator using extending and contracting deformation of the polymers, a driving force in the extending direction cannot be produced as it is since the polymer is in a film shape, and thus must be used with the terminal members 55a, 55b, which are arranged at both ends of the polymer film 52, connected by elastic bodies 59a, 59b that generate elastic force in the extending direction, and with a preload applied in the extending direction, as shown in the configuration of FIG. 7.
Patent document 1: Japanese Unexamined Patent Publication No. 11-169393
Non-patent document 1: Proceedings of SPIE, Vol. 4695, pp. 8-16
Non-patent document 2: SCIENCE, Vol. 287, No. 5454, pp. 836-839
The actuators of the above-described configuration have a drawback in that the performance lowers when forced displacement is externally applied. For example, when forced displacement is applied in the contracting direction to the actuator configured by one polymer film 52 as shown in FIG. 8A, such displacement cannot be received by the polymer of a film shape, and the polymer film 52 tends to bend as in FIG. 8B. The reference characters in FIGS. 8A and 8B indicate the members denoted with the same reference characters in FIG. 7. The polymer film 52 is likely to bend particularly at the connecting part of the terminal members 55a, 55b and at the intermediate portion of the polymer film 52. When repeatedly subjected to displacement, effects such as lowering of the strength of the polymer film 52 at the bent region appear and the performance of the actuator lowers. In the case of a stacked polymer actuator as well, the load tends to be applied in the direction of stripping the electrode or the electrolyte retention layer from the polymer film, and thus the bond between each layer weakens and the actuation efficiency lowers.
If forced displacement is applied to the actuator in the extending direction, on the other hand, irreversible deformation occurs to the polymer film. Although the polymer film itself has elasticity of a certain degree, the film tends to be irreversibly deformed or restoration from the extended state is not possible, and in the worst case, may break if pulled at a load of greater than or equal to a certain level. If irreversible deformation occurs, the movable range of the polymer actuator offsets by such an amount, and the operation same as that before the occurrence of the irreversible deformation cannot be performed.
In order to respond to such situation, on the assumption that the actuator will degrade, methods such as allowing a margin of performance, arranging the elastic element in series with the actuator, arranging a stopper against the actuator displacement, and the like are proposed. However, allowing a margin of performance on the assumption that the actuator will degrade is not desirable in terms of efficiency. Furthermore, a flexible elastic element is required to suppress the force applied to the actuator by the forced displacement if arranging the elastic element in series with the actuator. In this case, however, the output generated by the actuator is also not transmitted to the outside, and thus it is not desirable. Moreover, if the stopper is used against the actuator displacement, the allowable deformation amount differs between slow extension by self-actuation or the like and fast forced displacement by external force since the polymer exhibits viscoelastic behavior, and thus protection of the polymer and performance of the actuator cannot be obtained simultaneously simply by arranging the stopper with respect to the displacement of the actuator where both slow extension and fast forced displacement appear in combination.
Accordingly, in view of the above aspects, an object of the present invention is to provide a polymer actuator that can suppress lowering of performance when forced displacement is externally applied to the actuator without suppressing the performance of the actuator, a robot arm driven by the polymer actuator, and a robot equipped with the robot arm.